Separated by orbit

Astronaut Scott Kelly will embark on a 540-day space adventure. When he returns to earth, he will be poked and probed by a research team, who will compare the results with similar tests on his identical twin brother Mark

When Scott Kelly returns to space next year, he will be taking part in what may be the most far-flung twin study of all time. While circling the earth aboard the International Space Station for a full year - the longest single space adventure for any American astronaut - and after his return, scientists will closely monitor Commander Kelly to see what changes space has wrought. NASA has been studying the effects of long stays in space on astronauts for years, but this set of 10 investigations will be different: scientists will be doing the same poking, prodding and analysing on Commander Kelly's identical twin brother, Mark, a retired astronaut.

These in-depth studies could lead to a deeper understanding of the effects of living in space, which will become increasingly important as NASA plans missions to Mars or other destinations that could take years to complete.

"They have a very long-term vision," says Andrew Feinberg, a researcher at the Johns Hopkins University School of Medicine who is one of the investigators involved with the project. "It's kind of amazing."

Studies of twins are a favourite tool of scientists to understand environmental influences, and having the only identical twins to have flown in space as human guinea pigs opens physiological and psychological research opportunities for NASA. The space agency has budgeted $1.5 million for the 10 three-year studies, which were first proposed by the Kellys.

Feinberg said he would perform a full genome analysis of both men to study any epigenetic effects - that is, how the environment changes the genes and their function. Mark Kelly, who like his brother is a retired United States Navy captain, has been to space four times, as a shuttle pilot and commander, ending his last mission in 2011. He is married to former Congresswoman Gabrielle Giffords; after an attempt on her life in 2011 and her resignation from Congress the next year, they formed Americans for Responsible Solutions, an advocacy organisation working to keep guns out of the hands of the mentally ill, criminals and terrorists.

On a recent visit to The New York Times, the Kelly brothers, who are 50, show that whatever your accomplishments in this world or off it, you're still kind of a knucklehead when your brother is around. When the question of primacy is raised, Mark notes that he is six minutes older than his brother. But Scott is quick to point out that he was the first to go to space, "and I'll be the last." When a photographer asks one of them to stand slightly in front of the other for a portrait, they jokingly elbow each other to be the one out front.

Scott says that an extended expedition aboard the space station is easier now than in the past, with decent access to email and telephone communications as well as entertainment programming, exercise equipment and good air quality. (The old Russian space station Mir apparently was smelly, humid and mouldy.) Scott says that on his previous mission to the station, "I had a little bit of degradation" physically, "which is good." Knowing that his body is likely to change during this longer stay is promising, he says.

Their enthusiasm for the experiment has startled NASA scientists. "I volunteered to have them put a pressure probe in my skull," Scott says. "I thought it would be kind of cool to have a little bolt," he says, tapping a spot on his head. The scientists decided that it might cause problems, and demurred.

When he returns to earth, he says, he will have spent 540 days in space.

"I've had 55," Mark says.

"Fifty-four," Scott corrects him. "But who's counting?"

Scott said he was often asked the difference between a long flight and a short flight.

"The length," Mark deadpans. Deadpanning is a family trait.

Mark says that during his shorter flights, he felt that it was not possible to really adapt to the changes the body experienced in conditions of microgravity.

"After a 17-day shuttle flight, you don't feel 100 percent of anything."

Scott says that during his previous trip to the International Space Station, which began in 2010 and lasted six months, he felt he had fully adapted after the second month.

Mark predicts that this time, his brother would tire of life in space after a few months. "Halfway through, you're going to say, 'Okay, where's my ride?' "

© 2014 The New York Times